Fiber Bragg Grating (FBG) sensors have been used in temperature and strain sensor applications [A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C, G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol., vol. 15, pp. 1442-1462, August 1997]. One problem of FBG sensors is the discrimination of temperature and strain responses. For strain sensor applications, the wavelength shift of the FBG due to the applied strain should be measured, but the shift is also induced by environmental temperature perturbations. Therefore, it is necessary to subtract the temperature effect from the wavelength shift so that one can obtain the strain effect only.
A number of techniques for overcoming this limitation have been reported and demonstrated. For example, the dual wavelength technique involves writing two superimposed FBGs with large Bragg centre wavelength separation (850-1300 nm), which requires two broadband sources to address the sensors [M. G. Xu, J. L. Archambault, L. Reekie, and J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett., vol. 30, no. 13, pp. 1085-1087, 1994].
Cancellation of the thermal response of the gratings has been reported using two FBGs that are mounted on opposite sides of a bend surface, such that the gratings have equal, but opposite strain [M. G. Xu, J. L. Archambault, L. Reekie, and J. P. Dakin, “Thermally compensated bending gauge using surface mounted fiber gratings,” Int. J. Optoelectron, 9, pp. 281-283, 1994]. Light from a narrow bandwidth light source is split via a fiber coupler to the two FBGs mounted on opposite sides of the cantilever beam, and the light reflected from the respective FBGs is monitored utilizing an optical spectrum analyzer, for determining the difference in Bragg wavelength of the two FBGs for thermally-independent strain measurements.
Another example is a two grating sensor with different fiber diameters, which have the same temperature property, to discriminate temperature and strain induced wavelength shift [S. W. James, M. L. Dockney, and R. P. Tatam, “Simultaneous independent temperature and strain measurement using in-fiber Bragg grating sensors,” Electron. Lett., vol. 32, no. 12, pp. 1133-1134, 1996].
The above described sensors can discriminate the two effects, but their structures are complex. Some of the sensors need sophisticated equipment such as spectrum analyzers to detect wavelength changes or demodulators in order to convert the wavelength changes to power or current changes. These devices are usually expensive and the measurement speeds of these devices are usually limited by e.g. the scanning speed of tunable filters or tunable lasers. Commercially available Fabry-Perot (FP) filters or tunable lasers can only scan up to a maximum of 1 kHz may limit their application in high speed strain monitoring, e.g. blast induced strain monitoring cohere the frequency response may be up to MHz range.
A need therefore exists to provide an alternative technique to address at least one of the above mentioned problems.